feat: Multiple Adaptors per model class

[gselzer]

As I was working on #36, I found that VisPy does not allow multiple cameras to be active simultaneously on the same scene.

For scenex, this means that under the current framework we cannot view the same scene through multiple views - thus we have to resort to silly workarounds to get orthoviews.

My current thinking on a solution is to allow for multiple adaptors per model, which would require some added coordination that I'm not sure how we'd implement.

[tlambert03]

VisPy does not allow multiple cameras to be active simultaneously on the same scene.

yeah... there's something to be looked into here.
to be precise though (as I know you probably know), that comment there is specifically about a ViewBox... your comment here made me think "a scene can't have two cameras" ... which is not the case, but yes, as they say:

Multiple cameras can exist in one scene, although only one can be active at a time. A single camera can be used by multiple viewboxes at the same time.

this "one active at a time" does indeed seem like an extremely limiting situation, so much so that I'm surprised to hear that vispy itself doesn't have some better workaround. maybe ping the vispy gurus on the napari zulip to see if they know anything?

if it truly is the case that you can't view the same scene from multiple cameras without resorting to really clunky workarounds, i might actually consider it reason to consider ditching vispy... or just not supporting it for important things like multi-veiw